A high energy density secondary battery has been widely used as a power supply for small information equipment such as a portable phone and a notebook personal computer. The secondary batteries are often connected in series, the number of which corresponds to a voltage and current necessary for an objective apparatus or often used as an assembled cell in which they are connected in series or in parallel. Because a power supply voltage of the aforementioned small information equipment is about several V to 10V, the number of series connections in the assembled cell is 1 to 3.
On the other hand, in recent years, application of the secondary battery to purposes requiring a high power or high voltage such as household electric appliances, power tools, assisted bicycles and hybrid electric vehicles has been expanding rapidly as well as the power supply for the information equipment. Accompanied by this, the number of series connections in the assembled cell is increased, and it is not rare that 10 or more batteries are connected in series.
A prominent problem in connecting the batteries in series is a fluctuation between individual batteries (called cells). The fluctuation includes, for example, fluctuation in capacity, fluctuation in impedance, and fluctuation in the state of charge (SOC). A fluctuation which likely leads to an error of these ones is a fluctuation in voltage which is one of the fluctuations in the SOC.
If batteries having different capacities are connected in series or a plurality of batteries are connected under different SOCs, a cell having a higher voltage or a cell having a lower voltage than the average is generated in a fully charged state of the assembled cell. The cell having a higher voltage is turned into an overcharged state, whereby deterioration is intensified. If the charge is repeated, the cell whose deterioration is intensified by the overcharge has a reduced capacity, so that the overcharge is progressed, thereby accelerating the deterioration. As a result, the cycle service life of the assembled battery is shortened extremely than the service life of the cell.
Therefore, the assembled cell is demanded to control its cells not to be overcharged using a protective circuit. As well as the overcharging, all the cells of the assembled cell are preferred to be controlled not to be within abnormal ranges about other battery abnormalities such as over-discharging and overheating.
By the way, as a charge control method for use in nonaqueous electrolyte secondary battery, constant-current constant-voltage type is generally used. According to the constant-current constant-voltage type, the constant-current charge is carried out until the battery reaches a full-charge voltage, after which the constant-voltage charge is executed with the battery voltage maintained at a certain setting voltage. According to the constant-current constant-voltage type, when the cell is charged, the cell voltage never reaches an overcharge region. Even if a quick charge is carried out by increasing the charging current, a time required until the full-charge voltage is reached is shortened, but no higher voltage is reached.
On the other hand, if the assembled cell is charged according to the constant-current constant-voltage method, the fluctuation in voltage as described previously is produced. When a feedback control is carried out to a charging power supply, generally, the voltage of the assembled cell is controlled to be constant under the constant-voltage charge. Therefore, in any assembled cell in which cells having a low voltage and cells having a high voltage are connected in series, the cells having a high voltage reach the overcharge region.
Ideally, if the control for the constant-voltage charge can be carried out for all the cells of the assembled cell, the problem of overcharging due to fluctuation in voltage never occurs. However, to carry out the constant-voltage charge for the voltages of all the cells of the assembled cell, the following complicated processings need to be adopted into the feedback loop for controlling the charge power supply: (i) measuring the voltages of all the cells, (ii) selecting the highest cell voltage by comparing respective cell voltages, (iii) comparing the highest cell voltage with a reference voltage, and (iv) controlling the output voltage of the charge power supply based on the comparison result of (iii). Even if such a processing can be adopted into the feedback loop, the feedback loop has a number of unstable factors in terms of a response speed, stability and resistance to disturbance such as a noise. For this reason, a problem exists in reliability and as the number of cells connected in series in the assembled cell increases, the circuit configuration becomes complicated, which is disadvantageous in terms of cost and size.
In the assembled cell system for most applications, the assembled cell and the charger are provided separately and the assembled cell and the charger are connected only at the time of charging. However, complicated feedback information containing information about all the cell voltages cannot be transmitted between the assembled cell and the charger easily.
The fluctuation in voltage between the cells being recharged increases as the fluctuation in SOC increases between the cells or the voltage change rate of the cell voltage with respect to the SOC in the vicinity of the full charge increases or the charging current increases. Therefore, if it is attempted to charge rapidly the assembled cell composed of the cells having a characteristic which allows the cell voltage to rise steeply at an end period of recharging, particularly those problems become prominent. Because generally, the assembled cell has the problem about the fluctuation in voltage among the cells, a charge inhibit cell voltage is set on a higher voltage side than the full-charge voltage of the cell. However, if the fluctuation in voltage among the cells is increased, part of the cells surpass the charge inhibit cell voltage during recharging, so that sometime, the charging is stopped halfway.
If the assembled cell is charged according to the constant-current constant-voltage type in order to avoid such a phenomenon, the setting voltage at the time of constant voltage charging is set lower than (full-charge voltage of a cell)×(number of the cells connected in series). Generally, if the charging voltage is decreased further, the charge is executed in a region in which the voltage change with respect to the SOC is smaller. Thus, even if the same amount of the fluctuation in SOC is present, the fluctuation in voltage is decreased. Further, generally the voltage which the cell reaches is decreased, so that the voltage margin required until the charge inhibit cell voltage is reached can be increased. Even if the fluctuation in voltage occurs, that fluctuation is produced around voltages lower than the full-charge voltage which can be recharged on the cell, thereby lowering a possibility that overcharging may occur. However, if the setting voltage at the time of the constant voltage is lowered, a problem that the charge capacity is limited and a problem that the charge speed is lowered due to the limited quick charge performance are produced.
Another charge method which does not limit the charge capacity or charge speed while avoiding overcharging due to the fluctuation in voltage has been disclosed in, for example, JP-A 2005-151683 (KOKAI). According to the charge method disclosed in JP-A 2005-151683 (KOKAI), the entire assembled cell is charged according to the constant-current method and when any one of the cells contained in the assembled cell reaches the full-charge voltage, the charging current is decreased and then the constant-current charge is continued. By repeating this step to decrease the charging current step by step, the cells are charged.
According to this charge method, even if the voltage of the cell is fluctuated, the charging current is lowered when the cell which indicates the highest voltage reaches the full-charge voltage. It never happens that all the cell voltages drop and reach the overcharge voltage at a moment when the charging current drops. Further, if the constant current charge is carried out with a sufficiently small current value finally, a charge capacity similar to a case of the constant-current constant-voltage charge method can be obtained. Further, by setting the reduction rate of the charging current of each step smaller, a charge speed similar to the case of the constant-current constant-voltage method can be obtained.
According to the method disclosed in JP-A 2005-151683 (KOKAI), even if the voltage of the cell is fluctuated, the charge is continued. For this reason, even if cells having a much lower voltage than the other cells are contained, the charge is never stopped due to, for example, internal short-circuiting. The same thing happens in the case where part of the assembled cell is heated locally so that the impedance of part of the cells drops and then, the voltage of the cells at the time of charge is lowered.
According to the constant-current constant-voltage method, if there exists a cell having a particularly low voltage in the assembled cell due to the internal short-circuiting or overheating, the voltages of the other cells are necessarily increased. As a result, the charge is stopped because of the overcharge inhibit condition. However, the method disclosed in JP-A 2005-151683 (KOKAI) embraces a new problem that such a safety mechanism may be inactivated so that a cell exhibiting an internal short-circuiting problem is recharged and used as it is and consequently, the locally heated cell may be heated further by Joule heat produced by the rapid charging.